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92. Reverse Linked List II

Given the head of a singly linked list and two integers left and right where left <= right, reverse the nodes of the list from position left to position right, and return the reversed list.

Example 1:

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Input: head = [1,2,3,4,5], left = 2, right = 4
Output: [1,4,3,2,5]

Example 2:

Input: head = [5], left = 1, right = 1
Output: [5]

Constraints:

  • The number of nodes in the list is n.
  • 1 <= n <= 500
  • -500 <= Node.val <= 500
  • 1 <= left <= right <= n

Follow up: Could you do it in one pass?

Solution:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseBetween(ListNode head, int left, int right) {
        ListNode dummy = new ListNode(-1);

        dummy.next = head;

        ListNode p0 = dummy;

        for (int i = 0; i < left - 1; i++){
            p0 = p0.next;
        }

        ListNode pre = null;
        ListNode cur = p0.next;

        for (int i = 0; i < (right - left + 1); i++){
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }

        p0.next.next = cur;
        p0.next = pre;

        return dummy.next;

    }
}


// TC: O(n)
// SC: O(1)